Slot antenna having short radiating slots and long nonradiating distributed capacitance tuning slot



1963 HlROSHl MURATA ETAL 3,106,713

SLOT ANTENNA HAVING SHORT RADIATING SLOTS AND LONG NONRADIATINGDISTRIBUTED CAPACITANCE TUNING SLOT Filed Jan. 25, 1963 INVENTORSHires/n Nurafa Masao Maisqshiia M224 0 Ito M w m+M ATTORNEYS UnitedStates Patent 3,106,713 SLOT ANTENNA HAVING SHORT RADIATING SLOTS ANDLONG N ONRADIATING DISTRIB- UTED CAPACITANCE TUNING SLOT Hiroshi Murata,Yokohama City, Masao Matsushita, Hiratsuka City, and Mizuo Ito, YokohamaCity, Japan, assignors to The Furukawa Electric Company Limited, Tokyo,Japan, a corporation of Japan Filed Jan. 23, 1963, Ser. No. 253,490Claims priority, application Japan Jan. 26, 1962 6 Claims. (Cl. 343770)The present invention relates to a slot antenna which comprises atubular conductor for radiating from its side surface electromagneticwave.

In general, in Wireless communication with a moving station such asautomobile, train, etc., when the station passes through a valley,tunnel, etc., the electromagnetic wave is interrupted and cannot arriveat the station, thereby interrupting the communication. In such a case,therefore, it is necessary to provide the valley, tunnel, etc. with sucha guided line or lines to be coupled with the antenna of the movingstation as will transmit the electromagnetic wave to continue thecommunicamm.

For such purpose, use has been made of two parallel conductors or aleakage coaxial system which comprises a coaxial cable having an outerconductor which is provided in its longitudinal direction with acontinuous slot. It is impossible, however, to efficiently radiate theelectromagnetic wave in a direction at right angles with the line sincesuch system is influenced by other objects such as line supporters ortunnel wall near by, which fact results in an increase of thetransmission loss of the line.

In order to obviate the above disadvantage a tubular conductor isprovided on its side surface with a number of slots of suitable Widthand length arranged in a longitudinal direction and separated one fromanother with a desired interval, and the slots are brought intoresonance condition in order to radiate the electromagnetic wave fromthe tube wall. In this case, if the diameter of the tubular conductorbecomes extremely small as compared with the wave length, the feedingimpedance at the slots becomes too high to etficiently radiate theelectromagnetic wave. In other words, since it is necessary to satisfy acondition of D/ 0.12 (where D is the diameter of the tube and A is thewave length), the diameter or" the tube has to be large enough todecrease the feeding impedance. But, the use of the tubular conductorhaving a diameter greater than that necessary for the electromagneticwave transmission is extremely uneconomical.

The object of the invention is to obviate the above disadvantage andprovide a slot antenna which compn'ses a tubular conductor that radiatesfrom its side surface electromagnetic wave most eiiiciently even thoughthe diameter of the conductor becomes smaller than of the wave length.

For a better understanding of the invention, reference is made to theaccompanying drawings, of which FIG. 1 is a perspective view of a slotantenna embodying the invention;

FIG. 2 is its cross-sectional View;

FIG. 3 is a partial sectional view of a coaxial cable embodying theinvention;

FIGS. 4 and 5 show modified embodiments of a coaxial cable in accordancewith the invention; and

FIGS. 6, 7 and 8 are cross-sectional views of modified slit parts of thecoaxial cable according to the invention.

In FIG. 1, 1 designates a tubular conductor which is provided on its oneside surface with a number of A 3,166,713 Patented Get. a, 1963 slots ofabout /2 of the Wave length of the electromagnetic wave to betransmitted, which are separated one from another with an interval equalto about one wave length in a longitudinal direction. Each slot isconnected to a power source 3 to supply a high frequency voltage andexcited to radiate the electromagnetic wave in the conventional manner.In accordance with the invention, the tubular conductor 1 is provided atthe opposite side with a longitudinal slit 4 which is sufiicien-tly longas compared with the slot 2. The slit 4 provides, in case of a currentflowing circumferentially along the tube 1, a circuit having capacitancesuch as a distributed constant connected in parallel, so that theantenna is brought into a resonance condition even though there exists arelation of D/ 0.12 (where D is the diameter of the tube and A is thewave length of the electromagnetic wave to be transmitted). Thus, thefeeding impedance of the slot 2 is decreased and it is possible toelficiently radiate the electromagnetic wave.

FIG. 3 shows a coaxial cable embodying the invention having'an outertubular conductor 1 and a central conductor 5. The tubular conductor 1is provided on its one side with a number of slots 2 and on the oppositeside with a longitudinal slit 4 in the same manner as shown in FIG. 1.The central conductor 5 is supported in the outer tubular conductor 1 bymeans of insulating discs 6. 7 shows an outer protective sheath. In thisembodiment, the slots 2 are inclined at an angle 0 with respect to thelongitudinal axis. Thus, the slots 2 interrupt a current flowing throughthe tubular conductor '1 in the axial direction, so that a potentialdifference is produced across each side" edge of the slot 2, whichresults in radiation in transverse direction of the electromagnetic wavewithout any coupling element at the slot. The inclination of the slot 2may suitably be selected within a range of 0-i90 according to the energyof electromagnetic wave to be radiated, but it is generally selectedwithin a range of from 2 to 60. In the embodiments shown in FIGS. 1 and2, the length of slot 2 is usually made equal to about /2 of the Wavelength of electromagnetic wave, but it is not necessarily limited tosuch value, and may be selected at any length shorter than the wavelength. The interval between adjacent slots 2 may be determined inaccordance with the electromagnetic energy to be transmitted. case shownin FIG. 1, such interval may be made equal to about one to several wavelengths, while in the case shown in FIG. 2 such interval may be made oddmultiple of the A wave length. These intervals are not required to beequal to each other. One of the intervals may be made equal to two wavelengths, while the next interval to 1% 'wave length and the otherintervals may be made equal to one wave length. The length of the slit 4should be selected much longer than the length of the slot 2 on theopposite side. For example, the length of the slit 4 may be made 1.5-2.0times that of the slot 2. If the interval between the slots 2 is madefrom 1 to several times the A wave length, then the slit 4- may be madeto cover the total length of the tube.

FIG. 4 shows another embodiment of the coaxial tubular conductor whereinthe slots 2 are alternately sloped in opposite directions for the angleof +0 and 0 with the longitudinal axis. In this case, the internalbetween adjacent slots 2 is made odd multiple of the /2 wave length ofthe electromagnetic wave to be transmitted. Thus, the electromagneticwave can be radiated outwards from the slots in a similar manner to thatin the case of FIG. 3.

FIG. 5 shows a further embodiment wherein the slots 2 consists of twoportions 8 parallel with the longitudinal axis of the tube connected by:an intermediate inclined portion 9. A number of such slots 2 are formedon one side In the of the conductor 1 at a suitable interval, while theslit 4 is formed on the opposite side in the same manner as in the casesof 'FIGS. 1 to 4.

FIGS. 6 to 8 represent modifications of the slit 4 each edge of which iscapacitively coupled in order to decrease the feeding impedance at theslots. In case of FIG. 6, a :metal plate 10 is attached to each edge 11of the slit 4 in parallel, then the electrostatic capacity is increased.In case of FIG. 7, edges 11 are overlapped one upon another and adielectric piece 12 is inserted between the over-lapping edges 11. Inthe embodiment shown in FIG. 8, a metal strip 13 is attached to theedges 11 of the slit 4 with a spacer 12 made of dielectric material. Inany case, the electrostatic capacity at the slit is increased and thefeeding impedance at the slots is decreased, which results in anincrease of the radiation energy of the electromagnetic wave even thoughthe diameter of the tubular conductor is small.

As above mentioned, the invention provides a slot antenna comprising atubular conductor which is provided on its one side 'With short slotsand on the opposite side with a long longitudinal slit, and hasadvantages that the feeding impedance at the slots can be decreased evenif the diameter of the tubular conductor is made smaller than the wavelength of the electromagnetic wave to be transmitted, therebyeificiently radiating the electromagnetic wave, contrary to theconventional slot antenna provided with slots alone. The invention has aparticularly noteworthy advantage in that when it is applied to theouter conductor of a conventional coaxial cable used as the guided linein a tunnel, a valley, etc. Where the electric wave cannot reach, UHFand VHF communications with a moving station can be effected with highlyeflicient results.

What we claim is:

1. A slot antenna which comprises a tubular conductor, a number of slotsformed on one side of the conductor and separated with a desiredinterval, each of said slots having a length shorter than the wavelength of the electromag netic wave to be transmitted and a slit formedon the opposite side of the tubular conductor in a longitudinaldirection and having a length longer than that of the slot.

2. A slot antenna which comprises a coaxial conductor through the outertubular conductor, a number of slots formed on one side of said tubularconductor and separated with a desired interval, each of said slotshaving a length shorter than the wave length of the electromagnetic waveto be transmitted and a slit formed on the opposite side of theconductor in the longitudinal direction and having a length longer thanthat of the slot.

3. A slot antenna which comprises a tubular conductor, a number of slotsformed on one side of the conductor and inclined at an angle with alongitudinal axis and separated with a desired interval, each of saidslots having a length shorter than the wave length of theelectromagnetic wave to be transmitted and a slit formed on the oppositeside of the conductor in the longitudinal direction and having a lengthlonger than that of the slot.

4. A slot antenna which comprises a tubular conductor, a number of slotsformed on one side of the conductor, each inclined with a longitudinalaxis alternately at angles +6 and 0 and separated with an interval whichis about odd multiple of /2 Wave length of the electromagnetic wave tobe transmitted, each of said slots having a length shorter than the wavelength of the electromagnetic wave to be transmitted and a slit formedon the 0pposite side of the conductor and having a length longer thanthat of the slot.

5. A slot antenna which comprises a tubular conductor, a number of slotsformed on one side of the conductor and each consisting of portions inparallel with a longitudinal direction and a sloped portioninterconnecting said parallel portions and separated with a desiredinterval, each of said slots having a length shorter than the wavelength of the electromagnetic wave to be transmitted an a slit formed onthe opposite side of the conductor in the longitudinal direction andhaving a length longer than that of the slot.

6. A slot antenna which comprises a tubular conductor, a number of slotsformed on one side of the conductor and separated with a desiredinterval, each of said slots having a length shorter than the wavelength of the electromagnetic wave to be transmitted and a slit formedon the opposite side of the conductor and having a length longer thanthat of the slot, each edge of said slit being capaciti-vely coupled.

No references cited.

1. A SLOT ANTENNA WHICH COMPRISES A TUBULAR CONDUCTOR, A NUMBER OF SLOTSFORMED ON ONE SIDE OF THE CONDUCTOR AND SEPARATED WITH A DESIREDINTERVAL, EACH OF SAID SLOTS HAVING A LENGTH SHORTER THAN THE WAVELENGTH OF THE ELECTROMAGNETIC WAVE TO BE TRANSMITTED AND A SLIT FORMEDON THE OPPOSITE SIDE OF THE TUBULAR CONDUCTOR IN A LONGITUDINALDIRECTION AND HAVING A LENGTH LONGER THAN THAT OF THE SLOT.